CWE-127: Buffer Under-read

What is Buffer Under-read?

• Overview: Buffer Under-read occurs when a program reads data from a memory location before the start of a buffer. This can happen due to incorrect pointer arithmetic or using negative indices, leading to accessing unintended memory areas.

• Exploitation Methods:

• Attackers can exploit this vulnerability by crafting inputs that cause the program to read beyond the intended buffer limits.
• Common attack patterns include manipulating pointers or indices to access sensitive data or cause a program crash.

• Security Impact:

• Direct consequences include reading unintended memory, which may contain sensitive information.
• Potential cascading effects involve data corruption, application crashes, or exposure of sensitive data.
• Business impact includes data breaches, loss of customer trust, and potential legal consequences.

• Prevention Guidelines:

• Specific code-level fixes include validating index values and pointer calculations to ensure they remain within buffer bounds.
• Security best practices involve using safe buffer access functions and avoiding direct pointer arithmetic when possible.
• Recommended tools and frameworks include static analysis tools to detect buffer access issues and adopting languages or libraries with built-in bounds checking.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

#include <stdio.h>
#include <string.h>

void print_last_five_chars(const char* data) {
    int length = strlen(data);
    // Vulnerability: If length is less than 5, this will cause a buffer under-read
    for (int i = length - 5; i < length; i++) {
        printf("%c", data[i]);
    }
    printf("\n");
}

int main() {
    const char* test_data = "Hi"; // Example with less than 5 characters
    print_last_five_chars(test_data);
    return 0;
}

Explanation:

• Vulnerability: The function print_last_five_chars attempts to print the last five characters of the string. If the string's length is less than 5, the loop will access memory before the start of the string, causing a buffer under-read. This can lead to undefined behavior and potentially expose sensitive data or cause a crash.

How to fix Buffer Under-read?

To fix this vulnerability, ensure that the code verifies the length of the buffer before accessing it. Specifically, check if the buffer has at least five characters. If not, handle this case gracefully, such as by printing the entire string if it is shorter than five characters.

Fixed Code Example

#include <stdio.h>
#include <string.h>

void print_last_five_chars(const char* data) {
    int length = strlen(data);
    // Fix: Ensure there are at least 5 characters to read
    if (length < 5) {
        // If less than 5 characters, print the whole string
        printf("%s\n", data);
    } else {
        // Safe to access the last 5 characters
        for (int i = length - 5; i < length; i++) {
            printf("%c", data[i]);
        }
        printf("\n");
    }
}

int main() {
    const char* test_data = "Hi"; // Example with less than 5 characters
    print_last_five_chars(test_data);
    return 0;
}

Explanation:

• Fix: Added a condition to check if the length of the string is less than 5. If true, the program prints the entire string instead of trying to access potentially out-of-bounds memory. This ensures that the function will not attempt to read memory outside the bounds of the string.
• Best Practice: Always validate buffer sizes before accessing them, especially in low-level languages like C, to prevent accessing memory outside intended boundaries. This prevents buffer under-reads and potential undefined behavior. Additionally, ensure that any output or error handling is appropriate for the context in which the function is used.